1. Field of the Invention
The present invention pertains to antennas for transmitting and/or receiving electromagnetic radiation. More particularly, the present invention relates to apparatus for communicating circularly polarized radiation by means of linear radiation excitation elements arranged in coaxially arrayed cavities.
2. Description of Prior Art
The use of antennas for both transmitting and receiving electromagnetic radiation is well known. A variety of approaches to antenna construction is followed with a particular antenna design selected generally on the basis of several factors. These factors may include, among others, the frequency range in which the antenna is to function, physical size limitations on the antenna, mobility requirements, if any, the radiation pattern desired, and the nature and degree of the polarization of the radiation to be communicated. All of these factors may be critical when the antenna in question is mounted for operation on an aircraft or spacecraft. In such case, for example, the antenna must generally be compact and light weight, and able to function in a variety of orientations of the craft relative to the ground, and the station with which the craft is communicating.
In the case of spacecraft operating at high altitudes and communicating with a ground level station, polarization effects produced in wave fronts incident on boundaries between atmospheric layers can reduce signal strength significantly. Therefore, it is often desirable to conduct such radiation communication by utilizing circularly polarized radiation whereby the atmosphere-induced polarization effects are minimized.
Antennas constructed for flush mounting in spacecraft and other high flying craft are generally known to exhibit relatively narrow frequency bandwidth transmitting and receiving capabilities. Furthermore, to accommodate the flush mounting, such prior art antennas are generally constructed such that the spread of the effective radiation pattern is limited. Thus, to provide relatively uniform communication capability to a craft regardless of the craft orientation, the number of such antennas required to be arranged about the craft is increased.